For previous generations of electronics gurus, the 555 timer is somewhat of a sacred item. It is something that is incredibly useful and simple, and gets used in many projects (from the most basic to extremely complex). However, there are tons of people who came into electronics by building things with an Arduino and they have no idea this wonderful creation even exists. It’s one of those things that you’ll hear tossed around in jokes or snide remarks – “You could have just used a 555.” Thankfully, Adafruit just published a great guide on getting to know the 555 timer.
This guide goes into how a 555 timer works, and makes sure you fully understand what is going on with the two common use modes. It also has a pretty decent list of resources to keep you going to help you learn more.
One of the particularly useful resources is the “three fives” kit from EMSL that shows you how the internal workings of a 555 timer interact. And, of course, if you want that cute plushie, Adafruit sells that as well.